1. Field of the Invention
The invention relates to a method for determining the flow rate of a flowing medium according to the plummet principle. Furthermore, the invention relates to a device for determining the flow rate of a flowing medium according to the plummet principle with at least one plummet and at least one measuring tube through which the medium can flow, the plummet being movable along an axis in the measuring tube.
2. Description of Related Art
Measuring devices which determine the flow rate of a medium, especially of a fluid such as, for example, liquids or gases, by means of the plummet method, generally have a measuring tube which is located vertically—therefore, in the direction of the terrestrial gravitational field. The measuring tube through which the medium to be measured flows generally has a conical interior which widens in the flow direction. In the cone is a plummet which can move in the flow direction. The medium flows opposite the direction of action of the force of the terrestrial gravitational field, i.e., upward.
The flowing medium applies a force to the plummet in the flow direction which is dependent on the flow resistance of the plummet. Moreover, the buoyancy of the plummet acts in the flow direction. The force due to weight acts against the flow direction so that three forces altogether are acting on the plummet: two in the flow direction and one in the opposite direction. After a transient response the plummet remains at a height at which the three forces are just balanced. In this way, since the position, i.e., the vertical position of the plummet in the measuring tube, is dependent on the flow rate of the medium, this value allows a conclusion about the flow rate.
The height can be determined, for example, by the plummet being located in a viewing glass which is provided with a corresponding scale. Alternatively, as described in German Patent DE 196 39 060 C2, the plummet is connected to a permanent magnet.
The flow rate of the medium results from the determined height depending on a flow coefficient α. The flow coefficient α is, in turn, dependent on the geometrical shape of the measuring tube and that of the plummet as well as on the Ruppel number Ru which encompasses medium-specific parameters, such as viscosity and density (see, for example, German Utility Model DE 83 17 576 U1) and which is, however, not dependent on flow rate, in contrast to the Reynolds number. For implementation of the measurement principle in corresponding measurement devices, therefore specifically for each measurement device of the same construction, a data set with the flow coefficient α is determined as a function of the height h and the Ruppel number Ru for the medium which is to be measured and for a host of possible media—this yields the functional relationship α=f(h, Ru)—and is stored for evaluation. Here, each data set is dependent especially on the measured medium.
The disadvantage is that the determination of the calibration data sets is very complex, and thus, tedious and costly, since a measurement series must be carried out in particular for each medium.
Furthermore, it is known that the measuring tubes are located not only vertically, but also horizontally, the force due to weight being replaced by the spring force of a spring that acts on the plummet.
Since the calibration data sets which are necessary for the measurements are also dependent on the alignment of the measuring tube and the type of forces acting on the plummet, for a vertical arrangement of the measuring tube it is necessary to generate additional calibration data.